Refrigeration cycle apparatus

ABSTRACT

A refrigeration cycle apparatus ( 100 A) is provided with: a first compressor ( 1 ) including a first compression mechanism ( 11 ), an expansion mechanism ( 13 ), and a first motor ( 12 ); a second compressor ( 2 ) including a second compression mechanism ( 21 ), and a second motor ( 22 ); and a control device ( 6 ). The control device ( 6 ) reduces a rotation frequency of the second motor ( 22 ) at a reduction speed greater than a rotation frequency of the first motor ( 12 ) in a stop operation for stopping the first motor ( 12 ) and the second motor ( 22 ) while reducing the rotation frequencies of the first motor ( 12 ) and the second motor ( 12 ).

TECHNICAL FIELD

The present invention relates to a refrigeration cycle apparatus that isused for water heaters, air conditioners, and the like, and that isequipped with an expansion mechanism and compression mechanisms thereon.

BACKGROUND ART

Recently, a power recovery-type refrigeration cycle apparatus that usesan expansion mechanism instead of an expansion valve has been proposedfor further enhancing the efficiency of the refrigeration cycleapparatus. In this apparatus, the expansion mechanism recovers, aspower, the pressure energy produced in the process where a refrigerant(working fluid) expands, so that the electric power required to drive acompression mechanism should be reduced by the recovered amount. Such arefrigeration cycle apparatus uses an expander-compressor unit in whicha motor, a compression mechanism, and an expansion mechanism are coupledto each other by a shaft.

In the expander-compressor unit, the compression mechanism and theexpansion mechanism are coupled to each other by the shaft. Therefore,there are cases where the displacement of the compression mechanism isinsufficient, or the displacement of the expansion mechanism isinsufficient, under certain operational conditions. In response to this,there also has been proposed a refrigeration cycle apparatus that uses asecond compressor, in addition to the expander-compressor unit, in orderto keep the COP (Coefficient of Performance) of the refrigeration cycleapparatus high by ensuring the recovery of power even under suchoperational conditions (see Patent Literature 1, for example).

FIG. 13 is a diagram showing the configuration of the refrigerationcycle apparatus described in Patent Literature 1. As shown in FIG. 13,the refrigeration cycle apparatus using an expander-compressor unit 220and a second compressor 230 is provided with a refrigerant circuit 210and a controller 250 that serves as a control device. A firstcompression mechanism 221 of the expander-compressor unit 220 and asecond compression mechanism 231 of the second compressor 230 arearranged in parallel between an indoor heat exchanger 211 and an outdoorheat exchanger 212 in the refrigerant circuit 210. Further, the firstcompression mechanism 221 is coupled to a motor 222 and an expansionmechanism 223 by a shaft, and the second compression mechanism 231 iscoupled to the motor 232 by a shaft.

The controller 250 controls the second compressor 230 so that the highpressure of the refrigeration cycle should be a certain target value.Specifically, the controller 250 reduces the rotation frequency of themotor 232 if the measured value of a high pressure Ph exceeds the targetvalue, thereby reducing the discharge amount from the second compressionmechanism 231. On the other hand, the controller 250 increases therotational speed of the motor 232 if the measured value of the highpressure Ph falls below the target value, thereby increasing thedischarge amount from the second compression mechanism 231. Thus, it ispossible to maintain the high pressure Ph close to the target value,which makes it possible to operate the refrigeration cycle apparatuswhile keeping a high COP.

Meanwhile, when the operation of the refrigeration cycle apparatus isstopped, a large counter voltage might be generated in the drivingcircuit of the motor by suddenly stopping the motor. In order to preventthis, it can be employed, for example, to perform a stop operation inwhich the rotation frequency of the motor is reduced gradually, taking acertain time, and then the motor is completely stopped after therotation frequency has been reduced to some extent, as disclosed inPatent Literature 2.

CITATION LIST Patent Literatures

-   Patent Literature 1: JP 2004-212006 A-   Patent Literature 2: JP 58(1983)-99635 A

SUMMARY OF INVENTION Technical Problem

However, if the rotation frequencies of both motors 222 and 232 arereduced at the same reduction speed in the refrigeration cycle apparatususing the expander-compressor unit 220 and the second compressor 230 asshown in FIG. 13, the displacement of the expansion mechanism 223gradually becomes insufficient, and therefore the high pressure of therefrigeration cycle progressively shifts away from the optimal pressure(the pressure at which the COP is highest) that is defined correspondingto the low pressure thereof. Accordingly, the pressure differencebetween the high pressure and the low pressure in the refrigerationcycle is rendered difficult to be reduced, and thus a large energy isrequired to perform the stop operation. Here, it also is conceivable tocontrol the high pressure to be maintained at the optimal high pressureby measuring the high pressure. However, it is very difficult to performsuch control in the stop operation that is an unsteady operation.

In view of such circumstances, it is an object of the present inventionto achieve energy saving in the stop operation in a refrigeration cycleapparatus using an expander-compressor unit and a second compressor.

Solution to Problem

In order to achieve the above-mentioned object, the present inventionprovides a refrigeration cycle apparatus provided with: a firstcompressor including a first compression mechanism for compressing arefrigerant, an expansion mechanism for recovering power from therefrigerant that is expanding, and a first motor coupled to the firstcompression mechanism and the expansion mechanism by a shaft; a secondcompressor including a second compression mechanism, connected inparallel to the first compression mechanism in a refrigerant circuit,for compressing the refrigerant, and a second motor coupled to thesecond compression mechanism by a shaft; a radiator for radiating heatof the refrigerant discharged from the first compression mechanism andthe second compression mechanism; an evaporator for evaporating therefrigerant discharged from the expansion mechanism; and a controldevice for reducing the rotation frequency of the second motor at areduction speed greater than the rotation frequency of the first motorin a stop operation for stopping the first motor and the second motorwhile reducing the rotation frequencies of the first motor and thesecond motor.

Advantageous Effects of Invention

According to the above-mentioned configuration, the gradualinsufficiency of the displacement of the expansion mechanism can becompensated for by setting the reduction speed for the rotationfrequency of the second motor greater than the reduction speed for therotation frequency of the first motor. Therefore, according to thepresent invention, the pressure difference between the high pressure andthe low pressure in the refrigeration cycle can be reduced rapidly, andthus energy saving can be achieved in the stop operation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the configuration of arefrigeration cycle apparatus according to the first embodiment of thepresent invention.

FIG. 2 is a Mollier diagram showing a refrigeration cycle in a stopoperation in the first embodiment.

FIG. 3 is a graph showing the relationship between the time and therotation frequency of each of first and second motors in the stopoperation in the first embodiment.

FIG. 4 is a flow chart showing the stop operation in the firstembodiment.

FIG. 5 is a schematic diagram showing the configuration of arefrigeration cycle apparatus according to the second embodiment of thepresent invention.

FIG. 6 is a flow chart showing a stop operation in the secondembodiment.

FIG. 7 is a graph showing the relationship between the time and therotation frequency of each of first and second motors in a stopoperation in the third embodiment of the present invention.

FIG. 8 is a flow chart showing the stop operation in the thirdembodiment.

FIG. 9 is a schematic diagram showing the configuration of arefrigeration cycle apparatus according to the fourth embodiment of thepresent invention.

FIG. 10 is a flow chart showing a stop operation in the fourthembodiment.

FIG. 11 is a flow chart showing a stop operation in the fifth embodimentof the present invention.

FIG. 12 is a flow chart showing a stop operation in the sixth embodimentof the present invention.

FIG. 13 is a schematic diagram showing the configuration of aconventional refrigeration cycle apparatus.

DESCRIPTION OF EMBODIMENTS

Hereinafter, the embodiments of the present invention are described withreference to the drawings.

First Embodiment

FIG. 1 shows a refrigeration cycle apparatus 100A according to the firstembodiment of the present invention. This refrigeration cycle apparatus100A is provided with a refrigerant circuit 3 that allows a refrigerantto circulate therein. The refrigerant circuit 3 is composed of a firstcompressor (expander-compressor unit) 1, a second compressor 2, aradiator 4, an evaporator 5, and first to fourth pipes 3 a to 3 dconnecting these devices.

The first compressor 1 has a first closed casing 10 accommodating afirst compression mechanism 11, a first motor 12, and an expansionmechanism 13 that are sequentially coupled to one another by a firstshaft 15. The second compressor 2 has a second closed casing 20accommodating a second compression mechanism 21 and a second motor 22that are coupled to each other by a second shaft 25. The firstcompression mechanism 11 and the second compression mechanism 21 areconnected to the radiator 4 via the first pipe 3 a having two branchpipes that merge into one main pipe. The radiator 4 is connected to theexpansion mechanism 13 via the second pipe 3 b. The expansion mechanism13 is connected to the evaporator 5 via the third pipe 3 c. Theevaporator 5 is connected to the first compression mechanism 11 and thesecond compression mechanism 21 via the fourth pipe 3 d having one mainpipe that is divided into two branch pipes. That is, the firstcompression mechanism 11 and the second compression mechanism 21 arearranged in parallel in the refrigerant circuit 3. In other words, thefirst compression mechanism 11 and the second compression mechanism 21are connected in parallel in the refrigerant circuit 3.

The refrigerant compressed in the first compression mechanism 11 and therefrigerant compressed in the second compression mechanism 21 aredischarged respectively from the first compression mechanism 11 and thesecond compression mechanism 21 into the first pipe 3 a, and thenmerged, while flowing in the first pipe 3 a, to be introduced to theradiator 4. The refrigerants compressed in the compression mechanisms 11and 12 may be discharged from the compression mechanisms 11 and 12 onceinto the closed casings 10 and 20, and then exhausted from the closedcasings 10 and 20 into the first pipe 3 a. The refrigerant introduced tothe radiator 4 radiates heat in the radiator 4, and then introduced tothe expansion mechanism 13 through the second pipe 3 b. The refrigerantintroduced to the expansion mechanism 13 expands in the expansionmechanism 13. At this time, the expansion mechanism 13 recovers powerfrom the refrigerant that is expanding. The refrigerant that hasexpanded is discharged from the expansion mechanism 13 into the thirdpipe 3 c, and introduced to the evaporator 5. The refrigerant introducedto the evaporator 5 absorbs heat in the evaporator 5, and then isdiverged, while flowing in the fourth pipe 3 d, to be introduced to thefirst compression mechanism 11 and the second compression mechanism 21.

It is preferable that the displacement of the first compressionmechanism 11 is the same as that of the second compression mechanism 21.In this case, the same member can be used for constituting the firstcompression mechanism 11 and second compression mechanism 21, so thatthe cost can be reduced.

The refrigerant circuit 3 is filled with a refrigerant that reaches itssupercritical state on the high pressure side (the part extending fromthe first compression mechanism 11 and the second compression mechanism21 to the expansion mechanism 13 through the radiator 4). In thisembodiment, the refrigerant circuit 3 is filled with carbon dioxide(CO₂) as such a refrigerant. It should be noted that the type of therefrigerant is not particularly limited. The refrigerant may be arefrigerant that does not reach its supercritical state during operation(such as fluorocarbon refrigerants).

Further, the refrigeration cycle apparatus 100A is provided with acontrol device 6 that is equipped with a CPU and mainly controls therotation frequencies of the first motor 12 and the second motor 22. Thecontrol device 6 is connected to the first motor 12 and the second motor22, respectively, via inverters 61 and 62.

Upon receiving a stop signal in a continuous steady operation, forexample, when a stop switch is actuated by a user, the control device 6performs a stop operation in which the first motor 12 and the secondmotor 22 are stopped while the rotation frequencies of the first motor12 and the second motor 22 are reduced.

FIG. 2 is a Mollier diagram showing the refrigeration cycle in the stopoperation in the first embodiment. In FIG. 2, a denotes therefrigeration cycle immediately after the start of the stop operation, bdenotes the refrigeration cycle in the middle thereof, and c denotes therefrigeration cycle immediately before the end thereof. Also in FIG. 2,the points A, A′, and A″ indicate the state of the refrigerant flowingin the fourth pipe 3 d (that is, the refrigerant to be drawn into thecompression mechanisms), the points B, B′, and B″ indicate the state ofthe refrigerant flowing in the first pipe 3 a (to be precise, in themain pipe of the first pipe 3 a), the points C, C′, and C″ indicate thestate of the refrigerant flowing in the second pipe 3 b (that is, therefrigerant to be drawn into the expansion mechanism), and the points D,D′, and D″ indicate the state of the refrigerant flowing in the thirdpipe 3 c. As shown in FIG. 2, the pressure difference between the highpressure and the low pressure in the refrigeration cycle changes to bereduced as the stop operation proceeds, and therefore the refrigerationcycle shows a change in appearance as if it were shrinking.

If the rotation frequency of the first motor 12 and the rotationfrequency of the second motor 22 are reduced at the same reductionspeed, the density ratio between the refrigerant to be drawn into theexpansion mechanism and the refrigerant to be drawn into the compressionmechanism decreases as the stop operation proceeds. Thus, thedisplacement of the expansion mechanism 13 gradually becomesinsufficient. As a result, the pressure difference between the highpressure and the low pressure in the refrigeration cycle is rendereddifficult to be reduced.

In contrast, in this embodiment, the control device 6 reduces therotation frequency of the second motor 22 at a reduction speed greaterthan the rotation frequency of the first motor 12, as shown in FIG. 3.In this embodiment, a braking time Tf within which the first motor 12and the second motor 22 are to be stopped is set in advance, and thebraking time Tf is stored in the memory of the control device 6. Thebraking time Tf, for example, is one minute. Then, the control device 6completely stops the first motor 12 and the second motor 22simultaneously on the basis of the braking time Tf.

It should be noted that, although FIG. 3 illustrates a state where therotation frequency of the first motor 12 and the rotation frequency ofthe second motor 22 in the steady operation are the same, forconvenience of description, they are adjusted by the control device 5 toappropriate rotation frequencies so that the high pressure of therefrigeration cycle should be optimal.

Hereinafter, the control of the stop operation by the control device 6is described in detail with reference to the flow chart of FIG. 4.

First, the control device 6 waits to receive a stop signal (NO in stepS1), and upon receiving a stop signal (YES in step S1), it determines areduction speed X for the first motor and a reduction speed Y for thesecond motor (step S2). In this regard, the reduction speed Y is greaterthan the reduction speed X. For example, the reduction speed X for thefirst motor is 1 Hz/sec, and the reduction speed Y for the second motoris 2 Hz/sec.

Various methods can be employed to determine the reduction speeds X andY. For example, the following method can be employed. A table showing acorrespondence between the rotation frequencies and the reduction speedsat the start of the stop operation is stored in the memory of thecontrol device 6 in advance, and upon receiving a stop signal, thecontrol device 6 reads the reduction speeds that correspond respectivelyto the rotation frequencies of the first motor 12 and the second motor22 from the memory to determine the reduction speeds X and Y.Alternatively, the following method can be employed. The reductionpercentages for the rotation frequencies during the stop operation areset in advance, and the rotation frequencies at the time of thereception of the stop signal multiplied by the percentages are dividedby the braking time Tf to determine the reduction speeds X and Y.

Subsequently, the control device 6 reduces the rotation frequency of thefirst motor 12 at the reduction speed X, and reduces the rotationfrequency of the second motor 22 at the reduction speed Y. Then, thecontrol device 6 continues to reduce the rotation frequencies until theelapsed time T from the reception of the stop signal becomes equal to ormore than the braking time Tf stored in the memory (NO in step S4). Oncethe elapsed time T has become equal to or more than the braking time Tf(YES in step S4), it completely stops the first motor 12 and the secondmotor 22 (step S5).

In the refrigeration cycle apparatus 100A described above, the gradualinsufficiency of the displacement of the expansion mechanism 13 can becompensated for by setting the reduction speed Y for the rotationfrequency of the second motor 12 greater than the reduction speed X forthe rotation frequency of the first motor 12. Accordingly, the pressuredifference between the high pressure and the low pressure in therefrigeration cycle can be reduced rapidly, and thus energy saving canbe achieved in the stop operation.

Further in this embodiment, the first motor 12 and the second motor 22are completely stopped on the basis of the braking time Tf that has beenset in advance, and therefore the above-mentioned effects can beobtained with a simple and easy configuration.

For example, as shown in the following Table 1, under the condition ofX=1.0 Hz/sec and Y=2.0 Hz/sec, the pressure difference between therefrigerant to be drawn into the expansion mechanism and the refrigerantto be drawn into the compression mechanism can be reduced, when thedensity ratio thereof decreases.

TABLE 1 Change in the state of the refrigerant in the stop operation (X= 1.0 Hz/sec, Y = 2.0 Hz/sec) Refrigerant to be drawn into Refrigerantto be drawn into expansion mechanism compression mechanism PressureTemperature Density Pressure Temperature Density Pressure Density (Mpa)(° C.) (kg/m³) (Mpa) (° C.) (kg/m³) difference ratio Immediately 12 40717 3.5 1 97 8.5 7.39 after the start In the 10 40 628 4.5 10 135 5.54.65 middle Immediately 8 40 278 5.5 18.5 180 2.5 1.54 before the end

In contrast, as shown in the following Table 2, under the condition ofX=Y=2.0 Hz/sec, the pressure difference between the refrigerant to bedrawn into the expansion mechanism and the refrigerant to be drawn intothe compression mechanism cannot be reduced so much, when the densityratio thereof decreases.

TABLE 2 Change in the state of the refrigerant in the stop operation (X= Y = 2.0 Hz/sec) Refrigerant to be drawn into Refrigerant to be drawninto expansion mechanism compression mechanism Pressure TemperatureDensity Pressure Temperature Density Pressure Density (Mpa) (° C.)(kg/m³) (Mpa) (° C.) (kg/m³) difference ratio Immediately 12 40 717 3.51 97 8.5 7.39 after the start In the middle 11.5 40 702 3.7 3 104 7.86.75 Immediately 11.2 40 691 3.9 5 111 7.3 6.23 before the end

In order to reduce the pressure difference between the refrigerant to bedrawn into the expansion mechanism and the refrigerant to be drawn intothe compression mechanism more rapidly, the reduction speed Y ispreferably at least 1.5 times the reduction speed X, more preferably atleast 2.0 times the reduction speed X. In this case, it is possible toshorten the time to be taken for the stop operation, thus improving thereliability of the first compressor 1 and the second compressor 2.Further, in view of improving the stability of the temperature and thepressure in the refrigeration cycle apparatus, the reduction speed Y ispreferably 2.5 Hz/sec or less, more preferably 2.0 Hz/sec or less.

Second Embodiment

Next, FIG. 5 shows a refrigeration cycle apparatus 100B according to thesecond embodiment of the present invention. In this embodiment, the samecomponents (including the steps in the flow chart) as those in the firstembodiment are denoted by the same reference numerals, and thedescriptions thereof are omitted. This can be applied similarly to thethird to sixth embodiments to be described later.

The refrigeration cycle apparatus 100B of this embodiment is providedwith a pre-expansion temperature sensor 82 for detecting the temperatureof the refrigerant flowing in the second pipe 3 b, a high pressure sidepressure sensor 72 for detecting the pressure of the refrigerant flowingin the second pipe 3 b, a pre-compression temperature sensor 81 fordetecting the temperature of the refrigerant flowing in the fourth pipe3 d, and a low pressure side pressure sensor 71 for detecting thepressure of the refrigerant flowing in the fourth pipe 3 d. In thisembodiment, the high pressure side pressure sensor 72 is provided on thesecond pipe 3 b, and the low pressure side pressure sensor 71 isprovided on the branch pipe on the first compression mechanism 11 sideof the fourth pipe 3 d. However, the high pressure side pressure sensor72 may be provided on the main pipe of the first pipe 3 a, and the lowpressure side pressure sensor 71 may be provided on the third pipe 3 c,or the main pipe or the branch pipe on the second compression mechanism21 side of the fourth pipe 3 d.

The control device 6 calculates, upon receiving a stop signal, thedensity ratio between the refrigerant to be drawn into the expansionmechanism and the refrigerant to be drawn into the compressionmechanism, from the temperature and the pressure detected by thepre-expansion temperature sensor 82 and the high pressure side pressuresensor 72, and the pre-compression temperature sensor 81 and the lowpressure side temperature sensor 71. Then, the control device 6determines the reduction speed X for the first motor and the reductionspeed Y for the second motor from the calculated density ratio.

Specifically, as shown in FIG. 6, the control device 6 performs stepsS11 to S14, instead of step S2 show in FIG. 4. That is, the controldevice 6 detects, upon receiving a stop signal (YES in step S1), thetemperature and the pressure of the refrigerant to be drawn into theexpansion mechanism, using the pre-expansion temperature sensor 82 andthe high pressure side pressure sensor 72, and detects the temperatureand the pressure of the refrigerant to be drawn into the compressionmechanism, using the pre-compression temperature sensor 81 and the lowpressure side temperature sensor 71 (step S11). Subsequently, thecontrol device 6 calculates, from the temperature and the pressuredetected above, the density ratio between the refrigerant to be drawninto the expansion mechanism and the refrigerant to be drawn into thecompression mechanism (step S12).

Thereafter, the control device 6 calculates a target rotation frequencyH for the second motor 22 on the basis of the density ratio calculatedabove, and determines the reduction speed Y for the second motor (stepS13). Here, the target rotation frequency H is a rotation frequency todefine the degree to which the rotation frequency of the second motor 22should be reduced in the stop operation before the second motor 22 iscompletely stopped. For example, the target rotation frequency H may befound as follows. A value, for each specific density ratio, at which thepressure difference between the high pressure and the low pressure canbe sufficiently reduced is stored beforehand in the control device 6, asshown in the following Table 3, and the value corresponding to thedensity ratio calculated in step S12 can be obtained from such data.

TABLE 3 Density ratio 7.0 6.0 5.0 4.0 3.0 Target rotation frequency H[Hz] 65 60 55 50 45

In this embodiment, the first motor 12 and the second motor 22 arecompletely stopped (step S4) after the braking time Tf has elapsed fromthe reception of the stop signal, in the same manner as in the firstembodiment. Therefore, after the target rotation frequency H iscalculated, the reduction speed Y is determined according to Y=(therotation frequency at the time of the reception of the stopsignal−H)/Tf.

After the reduction speed Y for the second motor is determined, thereduction speed X for the first motor is determined so as to be lowerthan the reduction speed Y (step S14). For example, the reduction speedX can be calculated by subtracting a speed difference that has been setin advance from the reduction speed Y.

After determining the reduction speeds X and Y, the control device 6performs steps S3 to S5 in the same manner as in the first embodiment.

As described above, in this embodiment, the density ratio between therefrigerant to be drawn into the expansion mechanism and the refrigerantto be drawn into the compression mechanism is calculated, and then thereduction speed X for the first motor and the reduction speed Y for thesecond motor are determined from the thus calculated density ratio.Therefore, an appropriate stop operation based on the density ratio inthe steady operation can be performed, thus allowing further energysaving to be achieved.

In this embodiment, the reduction speed Y is determined using the targetrotation frequency H for the second motor 22 (step S13). However, forexample, as are the cases of the fourth to sixth embodiments to bedescribed later, if the first motor 12 and the second motor 22 arecompletely stopped without being based on the braking time Tf, thereduction speeds X and Y may be determined from the calculated densityratio, using the table showing the correspondence between the densityratio and the reduction speed.

Third Embodiment

Next, the third embodiment of the present invention is described. Therefrigeration cycle apparatus of this embodiment has the sameconfiguration as the refrigeration cycle apparatus 100A of the firstembodiment shown in FIG. 1, and therefore the diagram showing theconfiguration thereof is omitted.

This embodiment differs from the first embodiment only in the controlperformed by the control device 6. Specifically, in this embodiment, thefirst braking time Tf within which the first motor 12 is to be stoppedand the second braking time Tp within which the second motor 22 is to bestopped are set in advance, and these braking times Tf and Tp are storedin the memory of the control device 6. The second braking time Tp is setshorter than the first braking time Tf. The first braking time Tf, forexample, is one minute, and the second braking time Tp, for example, is30 seconds. Then, the control device 6 completely stops the second motor22 prior to the first motor 12 on the basis of the braking times Tf andTp, as shown in FIG. 7.

That is, as shown in FIG. 8, the control device 6 performs the samecontrol as in the first embodiment up to step S3, and continues toreduce the rotation frequencies of the first motor 12 and the secondmotor 22 until the elapsed time T from the reception of the stop signalbecomes equal to or more than the second braking time Tp stored in thememory (NO in step S21). Then, once the elapsed time T has become equalto or more than the second braking time Tp (YES in step S21), itcompletely stops the second motor 22 (step S22).

Thereafter, the control device 6 further continues to reduce therotation frequency of the first motor 12 until the elapsed time T fromthe reception of the stop signal becomes equal to or more than the firstbraking time Tf stored in the memory (NO in step S23). Once the elapsedtime T has become equal to or more than the first braking time Tf (YESin step S23), it completely stops the first motor 12 (step S24).

In this way, the second motor 22 is completely stopped prior to thefirst motor 12, resulting in an improvement in the safety.

Further, when the temperature sensors 81 and 82, and the pressuresensors 71 and 72 are provided, as described in the second embodiment,it also is possible to calculate, from the temperature and the pressuredetected by these sensors, the density ratio between the refrigerant tobe drawn into the expansion mechanism and the refrigerant to be drawninto the compression mechanism so as to determine the reduction speed Xfor the first motor and the reduction speed Y for the second motor fromthe calculated density ratio. For example, in the case where steps S11to S14 shown in FIG. 6 are employed instead of step S2, the rotationfrequency obtained by subtracting the target rotation frequency H fromthe rotation frequency at the time of the reception of the stop signalmay be divided by the second braking time Tp so as to determine thereduction speed Y for the second motor.

Fourth Embodiment

Next, FIG. 9 shows a refrigeration cycle apparatus 100C according to thefourth embodiment of the present invention. The refrigeration cycleapparatus 100B of this embodiment is provided with an evaporationtemperature sensor 83 for detecting an evaporation temperature Te of therefrigerant in the evaporator 5. Then, the control device 6 completelystops the first motor 12 and the second motor 22 simultaneously on thebasis of the evaporation temperature Te detected by the evaporationtemperature sensor 83.

That is, as shown in FIG. 10, the control device 6 performs the samecontrol as in the first embodiment up to step S3, and thereafter detectsthe evaporation temperature Te of the refrigerant in the evaporator 5,using the evaporation temperature sensor 83 (step S31). A settemperature TE is stored in the memory of the control device 6 inadvance, and the control device 6 continues to reduce the rotationfrequencies of the first motor 12 and the second motor 22 until thedetected evaporation temperature Te becomes equal to or more than theset temperature TE (NO in step S32). Then, once the detected evaporationtemperature Te has become equal to or more than the set temperature TE(YES in step S32), it completely stops the first motor 12 and the secondmotor 22 (step S5).

In this way, the low pressure of the refrigeration cycle can bepredicted from the evaporation temperature Te, and therefore the firstmotor 12 and the second motor 22 can be stopped after the pressuredifference between the refrigerant to be drawn into the expansionmechanism and the refrigerant to be drawn into the compression mechanismhas been reduced for sure. This allows the reliability of the firstcompressor 1 and the second compressor 2 to be improved.

Further, when the temperature sensors 81 and 82, and the pressuresensors 71 and 72 are provided, as described in the second embodiment,it also is possible to calculate, from the temperature and the pressuredetected by these sensors, the density ratio between the refrigerant tobe drawn into the expansion mechanism and the refrigerant to be drawninto the compression mechanism so as to determine the reduction speed Xfor the first motor and the reduction speed Y for the second motor fromthe calculated density ratio.

Furthermore, it also is possible to completely stop the second motor 22prior to the first motor 12, in the same manner as in the thirdembodiment, by preparing two kinds of the set temperature TE, which isthe condition to determine the complete stop.

Fifth Embodiment

Next, the fifth embodiment of the present invention is described. Therefrigeration cycle apparatus of this embodiment has the configurationin which the high pressure side pressure sensor 72 shown in FIG. 5 isadded to the refrigeration cycle apparatus 100A of the first embodimentshown in FIG. 1, and therefore the diagram showing the configurationthereof is omitted. Then, the control device 6 completely stops thefirst motor 12 and the second motor 22 simultaneously on the basis ofthe pressure Pd detected by the high pressure side pressure sensor 72.

That is, as shown in FIG. 11, the control device 6 performs the samecontrol as in the first embodiment up to step S3, and thereafter detectsthe pressure Pd of the refrigerant to be drawn into the expansionmechanism, using the high pressure side pressure sensor 72 (step S41). Aset pressure PD is stored in the memory of the control device 6 inadvance, and the control device 6 continues to reduce the rotationfrequencies of the first motor 12 and the second motor 22 until thedetected the pressure Pd becomes equal to or less than the set pressurePD (NO in step S42). Then, once the detected the pressure Pd has becomeequal to or less than the set pressure PD (YES in step S42), itcompletely stops the first motor 12 and the second motor 22 (step S5).

In this way, the first motor 12 and the second motor 22 can be stoppedafter the pressure difference between the refrigerant to be drawn intothe expansion mechanism and the refrigerant to be drawn into thecompression mechanism has been reduced for sure. This allows thereliability of the first compressor 1 and the second compressor 2 to beimproved.

Further, when the temperature sensors 81 and 82, and the low pressureside pressure sensor 71 in addition to the high pressure side pressuresensor 72 are provided, as described in the second embodiment, it alsois possible to calculate, from the temperature and the pressure detectedby these sensors, the density ratio between the refrigerant to be drawninto the expansion mechanism and the refrigerant to be drawn into thecompression mechanism so as to determine the reduction speed X for thefirst motor and the reduction speed Y for the second motor from thecalculated density ratio.

Furthermore, it also is possible to completely stop the second motor 22prior to the first motor 12, in the same manner as in the thirdembodiment, by preparing two kinds of the set pressure PD, which is thecondition to determine the complete stop.

Sixth Embodiment

Next, the fifth embodiment of the present invention is described. Therefrigeration cycle apparatus of this embodiment has the configurationin which the pre-compression temperature sensor 81 shown in FIG. 5 isadded to the refrigeration cycle apparatus 100C of the fourth embodimentshown in FIG. 9, and therefore the diagram showing the configurationthereof is omitted. Then, the control device 6 completely stops thefirst motor 12 and the second motor 22 simultaneously on the basis ofthe temperature difference AT between the temperature Ts detected by thepre-compression temperature sensor 81 and the temperature Te detected bythe evaporation temperature sensor 83, that is, superheat degree.

That is, as shown in FIG. 12, the control device 6 performs the samecontrol as in the first embodiment up to step S3, and thereafter detectsthe temperature Ts of the refrigerant to be drawn into the compressionmechanism, using the pre-compression temperature sensor 81 (step S51),as well as detecting the evaporation temperature Te of the refrigerantin the evaporator 5, using the evaporation temperature sensor 83 (stepS52). Subsequently, the control device 6 calculates the temperaturedifference ΔT according to ΔT=Ts−Te (step S53). A set superheat degreeSH is stored in the memory of the control device 6 in advance, and thecontrol device 6 continues to reduce the rotation frequencies of thefirst motor 12 and the second motor 22 until the calculated temperaturedifference ΔT becomes equal to or less than the set superheat degree SH(NO in step S54). Then, once the calculated temperature difference ΔThas become equal to or less than the set superheat degree SH (YES instep S54), it completely stops the first motor 12 and the second motor22 (step S5).

In this way, the first motor 12 and the second motor 22 can be stoppedbefore liquid compression is performed in the first compressionmechanism 11. This allows the reliability of the first compressor 1 andthe second compressor 2 to be improved.

Further, when the pressure sensors 71 and 72, and the pre-expansiontemperature sensor 82 in addition to the pre-compression temperaturesensor 81 are provided, as described in the second embodiment, it alsois possible to calculate, from the temperature and the pressure detectedby these sensors, the density ratio between the refrigerant to be drawninto the expansion mechanism and the refrigerant to be drawn into thecompression mechanism so as to determine the reduction speed X for thefirst motor and the reduction speed Y for the second motor from thecalculated density ratio.

Furthermore, it also is possible to completely stop the second motor 22prior to the first motor 12, in the same manner as in the thirdembodiment, by preparing two kinds of the set superheat degree SH, whichis the condition to determine the complete stop.

INDUSTRIAL APPLICABILITY

The refrigeration cycle apparatus of the present invention can be usedfor various applications such as bathroom drying and snow melting.

1. A refrigeration cycle apparatus comprising: a first compressorincluding a first compression mechanism for compressing a refrigerant,an expansion mechanism for recovering power from the refrigerant that isexpanding, and a first motor coupled to the first compression mechanismand the expansion mechanism by a shaft; a second compressor including asecond compression mechanism for compressing the refrigerant, the secondcompression mechanism being connected in parallel to the firstcompression mechanism in a refrigerant circuit, and a second motorcoupled to the second compression mechanism by a shaft; a radiator forradiating heat of the refrigerant discharged from the first compressionmechanism and the second compression mechanism; an evaporator forevaporating the refrigerant discharged from the expansion mechanism; anda control device for reducing a rotation frequency of the second motorat a reduction speed greater than a rotation frequency of the firstmotor in a stop operation for stopping the first motor and the secondmotor while reducing the rotation frequencies of the first motor and thesecond motor.
 2. The refrigeration cycle apparatus according to claim 1,wherein the control device determines, upon receiving a stop signal, areduction speed for the first motor and a reduction speed for the secondmotor, and reduces the rotation frequency of the first motor and therotation frequency of the second motor at the determined reductionspeeds.
 3. The refrigeration cycle apparatus according to claim 2,further comprising: a first pipe for introducing the refrigerant fromthe first compression mechanism and the second compression mechanism tothe radiator; a second pipe for introducing the refrigerant from theradiator to the expansion mechanism; a third pipe for introducing therefrigerant from the expansion mechanism to the evaporator; and a fourthpipe for introducing the refrigerant from the evaporator to the firstcompression mechanism and the second compression mechanism.
 4. Therefrigeration cycle apparatus according to claim 3, further comprising:a pre-expansion temperature sensor for detecting a temperature of therefrigerant flowing in the second pipe; a high pressure side pressuresensor for detecting a pressure of the refrigerant flowing in the secondpipe or the first pipe; a pre-compression temperature sensor fordetecting a temperature of the refrigerant flowing in the fourth pipe;and a low pressure side pressure sensor for detecting a pressure of therefrigerant flowing in the fourth pipe or the third pipe, wherein thecontrol device calculates, upon receiving a stop signal, a density ratiobetween the refrigerant flowing in the second pipe and the refrigerantflowing in the fourth pipe, from the temperature and the pressuredetected by the pre-expansion temperature sensor and the high pressureside pressure sensor, and the pre-compression temperature sensor and thelow pressure side pressure sensor, so as to determine a reduction speedfor the first motor and a reduction speed for the second motor from thecalculated density ratio.
 5. The refrigeration cycle apparatus accordingto any claim 1, wherein the control device completely stops the firstmotor and the second motor on the basis of a braking time that has beenset in advance.
 6. The refrigeration cycle apparatus according to claim1, further comprising: an evaporation temperature sensor for detectingan evaporation temperature of the refrigerant in the evaporator, whereinthe control device completely stops the first motor and the second motoron the basis of the evaporation temperature detected by the evaporationtemperature sensor.
 7. The refrigeration cycle apparatus according toclaim 3, further comprising: a high pressure side pressure sensor fordetecting a pressure of the refrigerant flowing in the first pipe or thesecond pipe, wherein the control device completely stops the first motorand the second motor on the basis of the pressure detected by the highpressure side pressure sensor.
 8. The refrigeration cycle apparatusaccording to claim 4, wherein the control device completely stops thefirst motor and the second motor on the basis of the pressure detectedby the high pressure side pressure sensor.
 9. The refrigeration cycleapparatus according to claim 3, further comprising: a pre-compressiontemperature sensor for detecting a temperature of the refrigerantflowing in the fourth pipe; and an evaporation temperature sensor fordetecting an evaporation temperature of the refrigerant in theevaporator, wherein the control device completely stops the first motorand the second motor on the basis of a temperature difference betweenthe temperature detected by the pre-compression temperature sensor andthe temperature detected by the evaporation temperature sensor.
 10. Therefrigeration cycle apparatus according to claim 4, further comprising:an evaporation temperature sensor for detecting an evaporationtemperature of the refrigerant in the evaporator, wherein the controldevice completely stops the first motor and the second motor on thebasis of a temperature difference between the temperature detected bythe pre-compression temperature sensor and the temperature detected bythe evaporation temperature sensor.
 11. The refrigeration cycleapparatus according to claim 5, wherein the control device completelystops the first motor and the second motor simultaneously.
 12. Therefrigeration cycle apparatus according to claim 5, wherein the controldevice completely stops the second motor prior to the first motor.